1. Field of the Invention
The present invention relates to reactor structural members, namely, materials used for constructing a reactor, resistant to corrosion in a reactor primary system of a nuclear power plant, and a method of suppressing corrosion of reactor structural members.
2. Description of the Related Art
In a BWR power plant, reactor water contains oxygen and hydrogen peroxide produced by the radiolysis of water in a radiation field. It is known that oxygen and hydrogen peroxide contained in the reactor water cause intergranular stress-corrosion cracking (IGSCC) in the structural members of stainless steels and nickel-base alloys in an environment of the high-temperature, high-pressure water of nuclear reactors. Initiation of stress-corrosion cracks and propagation of cracks are dependent on corrosion potential. Reduction of oxygen and hydrogen peroxide reduces the corrosion potential of a member. The lower the corrosion potential, the lower is the possibility of crack initiation and propagation of cracks.
A hydrogen injection method that injects hydrogen through a feedwater system into a nuclear reactor to reduce oxygen and hydrogen peroxide contained in reactor water is a method that has been practically applied to prevent the stress-corrosion cracking in some domestic and foreign nuclear power plants. However, the hydrogen injection method is attended with an adverse effect caused by the reaction of injected hydrogen with N-16 produced by nuclear reaction to produce volatile ammonia. The volatile ammonia is liable to enter the steam system, which increases the dose rate of the turbine system. When hydrogen is injected through the feedwater system into the reactor water, oxygen must be injected to reduce excess hydrogen in the off-gas system by recombination and various facilities are necessary therefor.
A corrosion potential reducing method recently proposed to reduce the corrosion potential of structural members without causing those problems adds a noble metal, such as platinum, to the reactor water to deposit the noble metal on the surfaces of the structural members and reduces the corrosion potential by injecting a small amount of hydrogen into the reactor water. This corrosion potential reducing method utilizes the property of the noble metal, such as platinum, to selectively arrest the reversible reaction of hydrogen having a low potential with an intention to reduce the corrosion potential by injecting a small amount of hydrogen into the reactor water.
However, when this method is practiced in a nuclear power plant, the noble metal adheres also to a zirconium oxide film contained in the fuel, which promotes the oxidation and hydrogenation of the fuel material. Moreover, the interaction of hydrogen injected into the reactor water and N-16 produced by nuclear reaction is promoted, volatile ammonia enters the steam system and the dose rate of the turbine system increases.
Furthermore, since a noble metal chemical containing impurities is used in a high concentration, the quality of the reactor water is deteriorated adversely affecting the soundness of the fuel materials. Thus, the noble metal injection method now in use exerts adverse effects on the maintenance of water quality, the reduction of radioactivity transition and the enhancement of the degree of burn-up of the fuel. To reduce such adverse effects, it is desired to develop a method that injects a small amount of the noble metal and a method that uses a substance other than the noble metal.
On the other hand, ions contained in feedwater adheres as loose particles to the surfaces of members disposed within the nuclear reactor when the feedwater has a high iron concentration. If the noble metal adhering to the structural members adheres to those particles, the noble metal adheres to the fuel when the particles separates from the surfaces of the structural members and promotes the oxidation and hydrogenation of the fuel materials.
As mentioned above, when hydrogen is injected into the reactor water by the known stress-corrosion cracking preventing method, the hydrogen and N-16 produced by nuclear reaction interact to produce volatile ammonia. The volatile ammonia thus produced is liable to enter the steam system to increase the dose rate of the turbine system. Various facilities are necessary to reduce excess hydrogen in the off-gas system by recombination.
When a noble metal is deposited on the surfaces of structural members by the method that adds the noble metal to the reactor water to reduce the corrosion potential by injecting a small amount of hydrogen into the reactor water, the noble metal adheres also to the zirconium oxide films and the oxidation and hydrogenation of the fuel materials are promoted. Further more, since a noble metal chemical containing impurities is used in a high concentration, the quality of the reactor water is deteriorated adversely affecting the soundness of the fuel materials.
Ion contained in the feedwater adheres in loose particles to the surfaces of members disposed within the nuclear reactor when the feedwater has a high iron concentration. If the noble metal adhering to the structural members adheres to those particles, the noble metal adheres to the fuel when the particles separate from the surfaces of the structural members.